In optical networks, wavelength division multiplexing (WDM) transmission systems are often used, e.g. in Synchronous Optical Networking (SONET)/Synchronous Digital Hierarchy (SDH) networks and Passive Optical Networks (PONs). The WDM transmission systems transmit a plurality of signals at a plurality of wavelengths that meet a multiplexer/demultiplexer grid. WDM transmission systems based on coherent light sources have been used, such as distributed feedback (DFB) lasers, which have different output wavelengths. Typically, a relatively large number of DFB lasers are needed for network operations and inventory, which increases cost. To reduce the amount of inventory, laser diodes that have tunable wavelengths may be used instead of the DFB lasers. However, because of the relatively high cost of the tunable laser diodes, the overall system cost may not be significantly reduced. To reduce the cost of WDM transmission systems, broadband incoherent light sources with spectrum-slicing schemes may be used instead of coherent light sources. The incoherent light sources include light emitting diodes (LEDs), free-running Fabry-Perot laser diodes (FPLDs), and sources based on amplified spontaneous emission (ASE) noise. The LEDs and FPLDs can be directly modulated, while external modulators are needed to modulate the ASE noise based sources. The spectrum-slicing of such incoherent light sources can introduce relatively strong noise and low data rates, which is not suitable for Gigahertz (GHz) communications.
Subsequently, low-cost high-speed WDM light sources have been introduced, such as using external seeding FPLDs or reflective semiconductor optical amplifiers (RSOAs). Such FPLDs or RSOAs use seeding light to emit substantially coherent light. The external seeding FPLDs and RSOAs are attractive because they can be used for colorless transmitters, where a plurality of such sources that are configured similarly may be coupled to a WDM filter and used to provide a plurality of distinct wavelengths in WDM networks. The colorless aspect of such sources reduces the amount of required inventory. However, such external seeding sources require seeding light based on either continuous wave (CW) or broadband incoherent light sources, such as spectrum-sliced ASE noise based sources, which increases cost.